For words that only have three letters, there is absolutely no 4th letter. One example is with the word “increase”, there aren't any extra letters afterwards and therefore must be the word "add". The word “addict” is while in the BIP39 wordlist however you would have applied “addi” for “addict”. https://fatallisto.com/story9535510/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition